Nurggor

Overview

WFB

Nurggorss are exactly what you'd expect. Full of disease, have bloated bellies and puss-filled sores giving them a highly asymmetrical shape, extra thick hide they can't feel, have one horn and a recurring image of threes in the details, wear the heaviest armor and can be seen poking through its rusted parts in places, and have no loss of vigor from it all. Unlike the rest of Nurgle's kin, Nurggors are not darkly cheerful in their happy hopelessness. Quite the opposite, they're possibly the most civilization-hating rabid monsters of all the Beastmen. For contrast the Tzaangors are kept organized by the direct whim of Tzeentch, Khorngors appreciate the discipline and craftsmanship their god centers around, and Slaangors hate themselves for loving the pleasures of ordered races. Nurgle's Beastmen just want to see the world rot, to an obsessive degree; if anything Nurggors represent an even more extreme version of the standard Beastmen hat trope. It helps quite a bit that most of the Beastmen named characters are clearly Nurggors (in actions and mindset anyway). In particular, that's pretty much all the Beastmen centered around in End Times.

Despite ALL THAT...lore states that they are more organized than most Beastmen. Khorngors get more organization, Nurggors get more armor. Both defy the trope of their species a small amount. Most likely the "organization" is more akin to Nurgle's disease-counting than actual military drills.

For most of WFB history they didn't really have their own stats, and existed as models (until they were squatted and replaced with generics for all varieties) that represented what Marks your forces had. In the original Realms of Chaos duology, when they were introduced in "The Lost and the Damned", Nurggors were defined by Hating foes aligned to Tzeentch and having a 50% chance to carry Nurgle's Rot, which didn't debilitate them but instead infected enemy units in close combat. Also, Nurggor shamans automatically had Nurgle spells appropriate to their level, selecting bonus spells normally.

In the Beasts of Chaos armybook, the only Nurggors were Bestigors, chariots (ridden by Bestigors), and Characters, since only they could take the Mark of Nurgle, presumably because they were the only ones to prove themselves. If the Mark was given to a Lord or Hero, then it would make them cause Fear and give them +1 Wound. Unsurprisingly this made it one of the more popular Marks if you were souping up a character with a ton of Wargear. If it was given to a unit then it made them cause Fear only, which was dogshit for Chariots, but fantastic on regular units because back then if you beat combat (easy enough for Bestigors to do) and you outnumbered your opponent (easy for Beastmen to do) then your opponent automatically failed their Break test. They could be Stubborn at Ld 10 with a re-roll and they'd still auto-fail it.

Variety was gutted when Chaos was split into three armies and somebody took Marks away from the Beastmen, then retconned the fluff to say that Beastmen were never given and could never have Marks.

40k

Beastmen mutations are not automatically Chaos in 40k, but at the same time Beastmen aren't a dominant and uncontrollable force so most Nurggors are just considered Chaos Mutants and lumped in with human Cultists. Way back in the day, when Chapters were first being created and released, each one would get a list of how many Beastmen slaves they had access to; obviously any Chapter of Chaos Space Marines that was devoted to Nurgle had Pestigors.

AoS

Nurggors have not appeared yet. Just like in End Times, being a Nurggor is just kind of how most Beastmen operate despite the new lore saying that most Beastmen bands have abandoned the worship of gods in favor of Chaos as a concept. But just like Khorngors, Nurggors are almost certain to eventually appear.